1. Field of the Invention
The present invention relates to a hydraulic control device for controlling hydraulic pressure of a torque converter with a lock-up clutch, with which an automatic transmission and the like mounted on a vehicle is provided.
2. Description of the Related Art
For example, as disclosed in Japanese Patent Application Publication No. 2007-211942 (hereinafter, referred to as “Patent Literature 1”), there is known a hydraulic control device for a torque converter (hereinafter, also occasionally abbreviated as a “TC”) with a lock-up clutch (hereinafter, also occasionally abbreviated as an “LC”), with which an automatic transmission or the like mounted on a vehicle is provided. The hydraulic control device disclosed in Patent Literature 1 controls supply and discharge of hydraulic oil to and from an oil chamber at a back pressure side of a hydraulic piston of the lock-up clutch. In the hydraulic control device of Patent Literature 1, a drain port for TC inner pressure arranged at an upper position than an outer diameter of the hydraulic piston is placed at a position higher than a drain port for LC back pressure, and this makes it possible to exert a stable canceller effect against centrifugal hydraulic pressure of the oil chamber at the back pressure, which increases at a high revolution of an engine or the like, thereby suppressing drag torque when the LC is caused not to engage. Moreover, by providing an oil reservoir in the drain port for the LC back pressure and supplying hydraulic oil decompressed to a pressure value to the value of atmosphere pressure by means of an orifice thereto, an oil passage from the oil chamber at the back pressure side to the drain port for LC back pressure is filled with the hydraulic oil, whereby preparation to operate the hydraulic piston of the LC is always made. Thus, it is possible to improve a hydraulic pressure response when the LC is operated, and this makes it possible to stabilize an LC function.
In this way, the prior art disclosed in Patent Literature 1 is characterized in that: a discharge end of the drain port for LC back pressure is placed above the upper end of the outer diameter of the hydraulic piston; the oil reservoir is provided at the discharge end of the drain port for LC back pressure; and hydraulic oil is always supplied thereto. However, in this prior art, an oil passage from the oil chamber at the back pressure to the discharge end of the hydraulic oil is long, and the oil passage is always filled with the hydraulic oil. For this reason, there has been a possibility that the hydraulic oil of the oil chamber at the back pressure side cannot be discharged rapidly when the lock-up clutch is switched from an engagement state to a non-engagement state. Thus, for example, there has been a possibility that engagement cannot be released immediately in the case where there is need to set the lock-up clutch to the non-engagement state quickly, such as at sudden braking of the vehicle (at so-called panic stop). In that case, although the vehicle is in substantially a stopped state, a driving shaft of the engine and a rotary shaft of the transmission become an engagement state, whereby it is impossible to cancel power transmitted from the engine. Therefore, there is a possibility that engine stall occurs. In particular, when D-N-R quick shift is carried out in a shift operation of the automatic transmission, a temporary stopped state occurs when to shift from forward to reverse. Despite this, since it is difficult to cause the lock-up clutch to set to a non-engagement state completely, the engagement state between the engine and the transmission continues. For that reason, it is impossible to cancel power transmitted from the engine, whereby there is a possibility that engine stall occurs.